Atomic elements consist of a nucleus that contains protons and neutrons. Protons carry a positive charge whilst neutrons are electrically neutral. In light elements with an atomic number less than 20, the neutron to proton ratio is generally equal to 1:1. This changes for heavier elements since the Coulomb interaction between many protons gets stronger and demands more neutrons for the nucleus to remain stable.
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when sodium metal is dropped in water, hydrogen gas in liberated due to extreme heat released as the reaction is exothermic, gas catches fire.